and —'------^~--------- = Volume of shrinkage water, in terms of the true clay volume,
expressed in per cent. In these formulas,
S = Specific gravity of clay, approaching the value 2.60 quite closely for all
clays of this type,
Wi = Weight of briquette in the wet state, Wz = Weight of dried briquette, Vi — Volume of briquette in wet state, and F2 = Volume of dried briquette.
Subtracting the volume of the shrinkage water from that of the total water, we can compute the volume of pore water. The ratio of the pore-water volume to the shrinkage-water volume is characteristic for the several types of clay. The lower this value, the more plastic is the material in question. For bonding clays in general, the ratio should not exceed 1:1.
A simple method for estimating the plastic character of clays consists in grinding them together with potters' flint in the proportion of 1:1, making them up to a plastic mass, and forming 1-in. cubes. After drying these thoroughly and immersing them in water, the time required to slake the clay down to a mud is characteristic of the structure of the clay. Heavy, "fat" clays will break down very much slower than lean and less plastic ones.
The mechanical strength of dried specimens of clay prepared from the unmixed material, or from a 1:1 mixture of clay and sand, is another means of estimating indirectly the plasticity and bonding power. This is especially true when the sand mixture is used, owing to the fact that some of the very "fat" clays are apt to check and crack in drying, without the addition of a non-plastic. The tensile strength is determined upon specimens made in the form of cement test briquettes which are broken in a suitable machine. The transverse strength test which is now made more commonly, employs bars 7 by 1 by 1 in. which, when dried, are ]oaded at the middle and broken. From the dimensions of the bar, the span and the load, the modulus of rupture is determined, expressed in pounds per square inch. The higher the strength, the greater should be the plasticity and the bonding power.
The fineness of the clay with reference to its finest particles, i.e., grains of the magnitude of 0.01 to 0.003 mm., is indicative in general of the content of the dispersed or colloidal portion, but is not necessarily a function of plasticity. Plasticity, therefore, is not merely a matter of fineness. The apparent specific gravity of the dried clay is sometimes useful in comparing different materials.
The Firing Behavior of Clays.—With reference to the behavior of a bond clay when heated to higher temperatures, several points must be noted. These are— the temperature at which the clay first becomes dense or vitrified, the temperature at which it overfires, and the point at which it shows evidence of fusion.
The overfiring point represents that condition at which the clay shows excessive vitrification and softening under pressure. Most clays at this point begin to evolve gases which cause the structure to become vesicular or spongy. This state usually marks the end of the usefulness of the clay by itself.
The firing behavior of a clay is determined by making up a series of briquettes, about 2 by IK by 1 in., placing them in a suitable kiln or furnace and drawing one or more specimens at different temperature intervals. It is necessary, of course, to maintain accurate temperature control by means of a pyrometer or by the use of standard pyrometric cones. It is customary to draw the first specimen at 1,050°C.